The present invention relates to novel flame-retardant resin compositions, and to insulated wires, shielded wires and insulating tubes comprising the composition.
Being different from light-emitting display devices such as cathode ray tubes and plasma display devices, liquid crystal display devices are generally provided with a light source (back light) at the back of the liquid crystal panel, in which the light emitted by the light source and transmitted through the panel is utilized for producing images on the panel. As the light source for the back light, generally used is a cold-cathode tube (fluorescent lamp) or the like, to which is applied a high-frequency current as generated in a lighting circuit to have a frequency of from tens to hundreds Hz or so and an effective voltage of hundreds volts or so.
The US UL (Underwriters Laboratories Inc.) Standards, which are the worldwide safety standards for insulated wires for internal wiring in electronic appliances, are applied to the insulating coatings for the insulated wires and shielded wires through which the high-frequency current is supplied to the cold-cathode tube, and to the insulating tubes to be applied over the connections between the cold-cathode tube and the insulated wires, for the purpose of ensuring the mechanical properties of those insulating coatings and insulating tubes and of ensuring the safety thereof against accidental fires. The UL Standards state the details of the initial and heat-aged mechanical properties (tensile strength and elongation), flame resistance, and withstand voltage characteristics of various insulated wires and insulating materials.
At present, the insulating materials for that use in insulated wires and others require flexibility for facilitating wiring work, in addition to the characteristics as stated in the UL Standards. In order to meet this requirement, for example, used are crosslinked compositions that are prepared by making soft polyolefinic resins, such as EVA (ethylene-vinylacetate copolymer) and EEA (ethylene-ethyl acrylate copolymer), resistant to flames followed by exposing the resulting flame-retardant resins to accelerated electron beam, and also silicone rubbers, etc. The degree of flexibility of those insulating materials is about 10 kg/cm2 or lower in terms of the modulus of tensile elasticity.
For insulated wires comprising insulating materials of those flame-retardant, crosslinked compositions of soft polyolefinic resins, the UL Standards state that the initial tensile strength of the insulating coatings of the insulated wires must be not smaller than 1500 psi (1.05 kg/mm2) and that the initial elongation thereof must be not smaller than 100%. Regarding the heat-aged physical properties of such insulated wires, the UL Standards state the retention of the initial values, while requiring that the heat-aged tensile strength of insulated wires must be not smaller than 70% of the initial one and that the heat-aged elongation thereof must be not smaller than 65% of the initial one. The heat-aging tests for the physical properties of insulated wires as stated in the UL Standards are, for example, such that samples for a temperature rating of 150xc2x0 C. are heated in a Geer oven aging tester at 180xc2x0 C. for 7 days, that samples for a temperature rating of 125xc2x0 C. are heated in the same at 158xc2x0 C. for 7 days, and that samples for a temperature rating of 105xc2x0 C. are heated in the same at 136xc2x0 C. for 7 days.
As well known, the current demand for compact and lightweight electronic appliances is extremely high. In order to satisfy that demand, it is necessary to provide small-sized internal units.
For example, in electronic appliances comprising liquid crystal display devices, not only the liquid crystal display units but also the peripheral devices of back light units are reduced in size and thinned, and the wiring space for insulated wires for cold-cathode tubes is being narrowed year by year. It is, therefore, important to reduce the diameter of insulated wires.
However, where insulated wires having a reduced diameter are used for high-density wiring in a limited wiring space, the amount of heat generated by the wired circuits increases. In such wired circuits, therefore, the insulating materials to be used require improved heat-aging resistance.
In addition, in such a narrow, limited wiring space, the insulated wires are often kept in contact with the chassis of the back light unit which is at the earth potential. In that case, leakage of high-frequency current from the insulating coatings will increase, often resulting in a great decrease in the supply voltage. As a result, a serious problem will occur that results in lowering the degree of luminance of the cold-cathode tube in the wired device.
For example, the current insulating materials, such as the flame-retardant compositions of soft polyolefinic compositions or silicone rubbers, have a high specific dielectric constant of 3.3 or higher. Therefore, if the insulating coating made of the current insulating material is used for producing insulated wires and if the thickness of the insulating coating is thinned in order to reduce the diameter of the insulated wires, the leak current from the insulated wires increases, often produces the serious problem.
In addition, as a result of the reduction in the wiring space, the insulated wires are often adjacent to the electronic parts in the wired units, in which the noise as derived from the high-frequency current running through the conductor parts will have some negative influences on the electronic circuits, thereby producing another problem of flickering of pictures.
In order to solve this problem, a structure comprising shielded wires may be employed in which the insulating coating formed over the core conductor is further covered with an outer conductor layer such as a metal braided conductor layer or a spirally-covered shielding layer. However, even in those shielded wires, the thickness of the insulating coating must be reduced in order to reduce the diameter of the shielded wires. Therefore, if the conventional insulating material having a high specific dielectric constant is used as the insulating coating in those shielded wires, the capacitance between the core conductor and the outer conductor layer is increased, thereby causing still another problem in that the rising of the wave pattern of the high-frequency current to be transmitted through the shielded wires becomes dull and that the supply voltage is lowered.
On the other hand, silicone rubbers have poor mechanical strength and, if used for producing shielded wires, they are problematic in that the insulating coating of the silicone rubber is cracked after having been rubbed with the metallic material constituting the outer conductor layer, whereby the core conductor and the outer conductor layer are often short-circuited.
For insulating tubes that may be formed over the outer conductor layer in order to insulate and protect the connections between the cold-cathode tube and the insulated wires therearound, generally used are insulating materials similar to those for the insulating coatings for the insulated wires, for example, flame-retardant compositions of soft polyolefinic resins such as those mentioned hereinabove. In those insulating tubes, therefore, it is also an important theme to lower the specific dielectric constant of those insulating materials.
The object of the invention is to provide a flame-retardant resin composition usable as an insulating material in producing insulated wires, shielded wires and insulating tubes that are especially favorably used in the technical field requiring high-frequency current transmission, for example, in the field of wiring of back lights for liquid crystal display devices, and also to provide insulated wires, shielded wires and insulating tubes comprising the composition.
In order to solve the problems mentioned hereinabove, we, the present inventors have found that a flame-retardant resin composition comprising:
(a) a thermoplastic polyolefinic resin having a density of smaller than 0.89 g/cm3, which is prepared by copolymerizing ethylene and a non-polar xcex1-olefin in the presence of a single-site metallocene catalyst;
(b) a halogen-containing flame retardant; and
(c) zinc oxide,
is effective. On the basis of this finding, we have completed the invention.
After having been crosslinked, the flame-retardant resin composition of the invention has the mechanical characteristics of an initial tensile strength of not smaller than 1.05 kg/mm2 and an initial elongation of not smaller than 100%, as stipulated in the UL Standards; has the flame resistance that satisfies all the following three requirements in the VW-1 perpendicular combustion test:
1) the longest burn time for the sample is within 60 seconds;
2) the kraft paper attached to the upper side of the sample is not burnt; and
3) the absorbent cotton disposed below the sample is not burnt by the burnt droppings of the sample;
has a modulus of tensile elasticity of not larger than 10 kg/cm2 to be highly elastic; and has good electric characteristics in that its specific dielectric constant is smaller than 3.3. Thus, the flame-retardant resin composition of the invention is good for solving the problems mentioned hereinabove.
As has been mentioned above, the flame-retardant resin composition of the invention has, after having been crosslinked, good mechanical properties, high flame resistance and high flexibility, while having a low relative dielectric constant. Therefore, the resin composition is favorably used, for example, for forming insulating coatings for insulated wires and shielded wires, for forming sheath layers for shielded wires, and for forming insulating tubes having or not having thermal shrinkability.
Preferably, the flame-retardant resin composition of the invention comprises, relative to 100 parts by weight of the thermoplastic polyolefinic resin (a) as produced by copolymerization of ethylene and a non-polar xcex1-olefin in the presence of a single-site metallocene catalyst to have a density of smaller than 0.89 g/cm3, from 20 to 60 parts by weight of the halogen-containing flame retardant (b) and from 5 to 20 parts by weight of the zinc oxide (c), in order to favorably satisfy its characteristic requirements mentioned above.
In the flame-retardant resin composition of the invention, the non-polar xcex1-olefin that constitutes the thermoplastic olefinic resin (a) is preferably at least one selected from the group consisting of 1-butene, 4-methyl-1-pentene, 1-hexene and 1-octene, since the mechanical characteristics of the composition are especially good and since the composition is highly flexible and has a lower specific dielectric constant.
The flame-retardant resin composition, as comprising the combination of the halogen-containing flame retardant (b) and the zinc oxide (c) along with the thermoplastic polyolefinic resin (a), is favorably formed into insulating coatings, sheath layers and insulating tubes such as those mentioned hereinabove, which stand the VW-1 perpendicular combustion test for their flame resistance.
In the flame-retardant composition of the invention, the halogen-containing flame retardant (b) is preferably at least one bromine-containing flame retardant selected from the group consisting of ethylenebis-bromophthalimides, bis(bromophenyl)alkanes, bis(bromophenyl)terephthalamides and polybromobiphenyl ethers. The resin composition comprising such a bromine-containing flame retardant is favorably formed into insulating coatings, sheath layers and insulating tubes such as those mentioned hereinabove, which have good heat-aging resistance as stipulated in the UL Standards. Precisely, after having been heat-aged, the insulating coatings, sheath layers and insulating tubes formed from the resin composition still have a retention of tensile strength of 70% or higher and a retention of elongation of 65% or higher.
The invention further provides insulated wires, which are produced by coating a conductor with the flame-retardant resin composition of the invention followed by crosslinking the composition to form an insulating coating over the conductor.
In the insulated wires of the invention, the insulating coating has a low density and is lightweight and flexible. Therefore, the insulated wires are favorably used for wiring back light units in liquid crystal display devices. In particular, the insulated wires can be well thinned and can be favorably used for high-density wiring even in a limited wiring space.
In addition, in the insulated wires of the invention, the insulating coating has good mechanical properties, good heat-aging resistance and good flame resistance, as stipulated in the UL Standards. Therefore, even when thinned, the insulated wires are still favorably applicable to high-density wiring even in a limited wiring space with high safety.
Moreover, the insulating coating in the insulated wires of the invention has a low specific dielectric constant. Therefore, even when the insulated wires are kept in contact with the chassis of a back light unit which is at the earth potential, little leak of high-frequency current from the insulating coating occurs, without producing the decrease in the degree of luminance of the cold-cathode tube in the wired device.
The invention still further provides shielded wires, which are produced by coating a conductor with the flame-retardant resin composition of the invention, then crosslinking the composition to form a foamed or non-foamed insulating coating over the conductor, and thereafter laminating an outer conductor layer and a sheath layer over the insulating coating in that order.
Like that in the insulated wires, the insulating coating of the shielded wires of the invention has a low density and is lightweight and flexible. Therefore, the shielded wires are favorably used for wiring back light units in liquid crystal display devices. In particular, the shielded wires can be well thinned and can be favorably used for high-density wiring even in a limited wiring space.
In addition, also like that in the insulated wires, the insulating coating of the shielded wires has good mechanical properties, good heat-aging resistance and good flame resistance, as stipulated in the UL Standards. Therefore, even when thinned, the shielded wires are still favorably applicable to high-density wiring even in a limited wiring space with high safety.
Moreover, since the insulating coating of the shielded wires has such good mechanical properties, it is not cracked even when rubbed with the metallic material constituting the outer conductor layer in the process of producing the shielded wires. Therefore, the shielded wires of the invention are surely out of the problem of short-circuiting between the core conductor and the outer conductor layer.
Furthermore, the insulating coating of the shielded wires of the invention has a low specific dielectric constant. Therefore, the shielded wires are free from the trouble of such that the capacitance between the core conductor and the outer conductor layer is increased thereby making the rising of the wave pattern of the high-frequency current to be transmitted through the shielded wires dull and lowering the supply voltage. In addition, owing to the function of the outer conductor layer, the core conductor in the shielded wires is electromagnetically shielded from any externals. Therefore, in electronic units, even when the shielded wires are kept adjacent to the other electronic parts, they are surely free from the trouble of such that the core conductor in the wires has some negative influences on the electronic circuits in the units to make pictures flicker. In particular, where the insulating coating in the shielded wires has a foamed structure, it shall have a lower specific dielectric constant than the insulating coating having a non-foamed structure. Therefore, the shielded wires of the invention in which the insulating coating is a foamed one shall have much more improved characteristics.
Preferably in the shielded wires of the invention, the sheath layer covering the outer conductor layer is formed from the flame-resistant resin composition of the invention by applying the composition onto the outer conductor layer followed by crosslinking it thereon, like the insulating coating.
The shielded wires of that preferred embodiment shall have much more improved flexibility, mechanical-properties, heat-aging resistance and flame resistance.
The invention still further provides insulating tubes, which are produced by shaping the flame-retardant resin composition of the invention into a tube followed by crosslinking it. If desired, the crosslinked insulating tubes may be expanded under heat in the radial direction and thereafter cooled in the expanded condition to thereby make the tubes have thermal shrinkability.
Like that in the insulated wires, the insulating coating in insulating tubes of the invention has a low density and is lightweight and flexible. Therefore, the insulating tubes are favorably used for covering the connections between a cold-cathode tube and insulated wires therearound even in a limited narrow space. In addition, also like that in the insulated wires, the insulating coating in the insulating tubes has good mechanical properties, good heat-aging resistance and good flame resistance as stipulated in the UL Standards. Therefore, when used for covering the connections between a cold-cathode tube and insulated wires therearound, the insulating tubes ensure high safety of the covered connections. Moreover, since the insulating tubes have a low specific dielectric constant, high-frequency current leaks out little through them, and the degree of luminance of the cold-cathode tube is not lowered.